Answer: The correct answer is - D) a red allele present on at least one of two homologous chromosomes (with genotype Rr).
Dominant trait (depicted by dominant allele, which is present in capital letter such as 'R' for red) is the one that masks the expression of recessive trait (depicted by small letter such as 'r' for white).
For a dominant trait to express, its genotype can be homozygous dominant ( that is having both the alleles as dominant RR) or heterozygous dominant (having both dominant and recessive allele such as Rr).
On the other hand, for a recessive trait to express, genotype must be homozygous recessive (such as rr for white flower).
In the given question, when a red flower (which could be Rr or RR genotype) is crossed with white flower (rr) and if it produces red flower, it will certainly have the heterozygous genotype that is Rr (as it will inherit 'R' allele from one parent and 'r' allele from another to become red flower).
Thus, a red allele (R) must be present on at least one of two homologous chromosomes.
